称取2.000 g猪肝样品，加入0.2 mol·L^-1 HCl溶液20 mL，在25℃下超声提取40 min，加入70%~72%（φ） HClO₄溶液200 μL，混合均匀后低温高速离心。取上清液，加入正己烷10 mL，振荡均匀，离心。弃去有机相，取10.00 mL提取液于已活化的MCX固相萃取柱上，用甲醇（1+1）溶液淋洗、氨水-甲醇（4+96）混合液洗脱。将洗脱液用氮气吹至近干，用甲醇定容至1 mL，经0.22 μm滤膜过滤。滤液经Waters ATLANTICS C₁₈色谱柱（2.1 mm×150 mm，5 μm）分离，以0.1%（φ）甲酸溶液和乙腈为流动相进行梯度洗脱。质谱分析中选择电喷雾离子源和多反应监测模式。12种β₂-受体激动剂的质量浓度在一定范围内与其对应的峰面积呈线性关系，检出限（3S/N）在0.01~0.1 μg·kg^-1之间。加标回收率为70.0%~120%，日内和日间测定值的相对标准偏差（n=6）小于12%。

The pig liver sample (2.000 g) was added into 20 mL of 0.2 mol·L^-1 HCl solution for ultrasonic extraction at 25℃ for 40 min. Then 200 μL of 70%-72% (φ) HClO₄ solution was added and mixed well. The mixture was centrifuged at low temperature and high speed. 10 mL of n-hexane was added into the supernatant and the mixture was shaken evenly, and centrifuged. The organic phase was discarded, and 10.00 mL of the extract was taken on an activated MCX solid phase extraction column, rinsed with methanol (1+1) sloution and eluted with a mixture of ammonia-methanol (4+96). The eluate was concerntrated to near dryness by nitrogen blowing, and made up to 1 mL with methanol. After filteration with 0.22 μm filter membrane, the filtrate was separated on a Waters ATLANTICS C₁₈ chromatographic column (2.2 mm×150 mm, 5 μm), with a mixture of 0.1% (φ) formic acid solution and acetonitrile as mobile phase for gradient elution. Electrospray ion source and multiple reaction monitoring mode were selected for mass spectrometry analysis. Linear relationships were found between the peak areas of 12 β₂-agonists with the corresponding mass concentration within definite ranges, and the detection limits (3S/N) were in the range of 0.01-0.1 μg·kg^-1. Values of recovery obtained by standard addition method were in the range of 70.0%-120%, and RSDs (n=6) of intra-day and inter-day were less than 12%.